Recently, optical communications has become one of the dominant forms of telecommunications. Further, multiple wavelengths are now often utilised to transmit information over an optical fibre network or the like. In particular, wavelength division multiplexed (WDM) and dense wavelength division multiplexed (DWDM) systems are becoming increasingly popular and are thought to be the best way forward for transmitting high volumes of information over a telecommunications network.
With any DWDM system, it is necessary to be able to extract a particular wavelength channel from a signal carrying multiple wavelengths. This is normally the function of an add drop multiplexer which, as its name suggests, adds or drops channels on demand. Many different add drop multiplexers are known with the basic requirement being that the add drop multiplexer can extract a particular wavelength from a series of wavelengths or insert a particular wavelength from a series of wavelengths.
The increasing demand for optical telecommunications has lead to a high demand for add drop multiplexers. In particular, a tunable add drop multiplexer, wherein the wavelength extracted can be tuned or altered is highly desirable. Further, preferably the tuning process is a non disruptive one in that the other signals travelling on other wavelengths via an add drop multiplexer do not experience any interference during the tuning process.